The present invention relates to a liquid-metal cooled fast neutron nuclear reactor having a vessel closed by a horizontal slab on which is suspended a core cover plug. In such a reactor, the invention more particularly relates to the core cover plug.
In fast neutron nuclear reactors, the reactor core is placed in a main vessel filled with liquid metal and generally constituted by sodium. This liquid metal is used for transferring to heat exchangers, the heat produced by the fission reaction occurring in the reactor core. The circulation of sodium between the core and the exchangers is assured by pumps.
In reactors of the integrated type, the heat exchangers and pumps are also placed within the main reactor vessel. An inner vessel then defines within the main vessel a hot collector containing the sodium leaving the reactor core and a cold collector containing the sodium leaving the heat exchangers.
In fast neutron nuclear reactors, the expression "core cover plug" is used for designating a group of parts suspended on the slab sealing the main vessel and located above the reactor core. This group or assembly comprises a plug sealing a central opening formed in the slab, a subassembly called the "core cover" and located immediately above the core, as well as the parts by which the core cover is suspended on the plug.
The core cover has the double function of making it possible to monitor and check the reactor core, as well as deflecting the jet of hot sodium leaving the core towards the heat exchangers.
In order to monitor the core, the core cover generally also comprises liquid sodium sampling tubes serving to detect and locate any fractures to cans in the reactor core, as well as smaller diameter tubes housing thermocouples.
In order to make it possible to check the fission reaction in the reactor core, the core cover plug has vertically axed, large diameter guide tubes or sleeves permitting the passage of control rods made from neutron-absorbing materials and vertical command bars on which the rods are normally suspended.
Finally, the deflection of the sodium jet from the reactor core towards the heat exchangers is obtained by means of one or more deflectors located in the immediate vicinity of the core.
Thus, in a fast neutron nuclear reactor, the core cover plug constitutes an important component with respect to the thermohydraulic behaviour of the sodium in the hot collector.
Thus, calculations show that the lower compartment of the core cover plug must have a high porosity, in order that the hot sodium jet leaving the core is deflected and channelled radially towards the heat exchangers through this compartment. Thus, an inadequate porosity leads to the passage of an important proportion of the sodium beneath the core cover plug, which disturbs the flow, particularly at the free surface of the sodium, which must remain as calm as possible.
The thermohydraulic behaviour of the sodium within the hot collector must limit to the greatest possible extent the thermal stressing to which the structures such as the internal vessel or the core cover plug are exposed. These thermal stresses must be relatively small in order to ensure the long-term mechanical behaviour of these structures.
These thermohydraulic conditions which have to be fulfilled in the hot collector become even more difficult to obtain with an increase in the compactness of said collector or in the liquid sodium flowrate in the core. These conditions are also more difficult to respect when a fuel assembly storage area is provided in the hot collector, when use is made of an internal vessel with a single step or when the hydrostatic charge between the upper plane of the core and the free level of the sodium is low.
French Pat. No. 2,289,031 describes a fast neutron nuclear reactor having a core cover plug mainly formed by an external ferrule which is perforated in its lower part and a certain number of perforated horizontal plates located immediately above the core.
Although such a structure can be used when the hereinbefore defined thermohydraulic conditions are not too difficult, its use is virtually impossible when these conditions are stricter, e.g. under the influence of one or more of the factors referred to hereinbefore. Thus, the perforations formed in the lower plates of the core cover plug define a porosity limit, which can, if exceeded, lead to a deterioration in the mechanical behaviour of the structure.
Comparable observations can be made with respect to the core cover plug described in French patent application 83 08734, in which conical or truncated cone-shaped deflecting plates are suspended on the sleeves, immediately above the core. Thus, even though the structure of this core cover plug is lighter than that described in the preceding document, which makes it possible to reduce its thermal inertia, this is carried out to the detriment of the porosity of the lower part of the core cover plug. Therefore disturbances can occur on the free surface of the sodium in the hot collector.